A thread forming tap is known that includes screw threads formed into a male thread shape with margin portions disposed to project in a radial direction and relief portions having a diameter smaller than the margin portions alternatively arranged and that causes the margin portions of the screw threads to cut into a surface layer portion of a prepared hole formed in a work to plastically deform the surface layer portion so as to form female threads. A technique for reducing a working torque and increasing a tool life is proposed for such a thread forming tap. For example, this corresponds to a torque reductive thread forming tap described in Patent Document 1. According to this technique, it is considered that a weak relief portion is disposed on a portion corresponding to the margin portions to realize a thread forming tap with a working torque reduced to extend a tool life.